This invention relates to a method and apparatus for producing monosize ceramic particles from organometallic liquid precursors in a continuous process.
A number of batch chemical methods and modifications thereto are currently used to produce high density ceramic refractory bodies (green bodies) for a multiplicity of industrial applications. Major emphasis has been placed on producing maximum refractory strength via monodisperse ceramic particles. Most of these batch processes are lengthy and costly.
The usual method of preparing a fine-particle oxide salts from oxalates, acetates, and carbonates is to thermally decompose, pyrolyze, or hydrolyze them to their oxides. Current results obtained using the controlled hydrolysis method are the most successful to date in producing accurately sized particles having spheroidal shape and a diameter of approximately 200 nanometers with low agglomeration of particles.
Size dispersion of particles no better than .+-.10 percent can be produced by either: (1) precipitation of supersaturated solutions of an appropriate water reactive metal alkoxide or (2) preparation of a monodisperse aerosol by condensation of a supersaturated atmosphere of organometallic vapor in a flowing inert gas stream. Upon dispersal in water to form a colloidal suspension, only disordered colloids, which have a tendency to aggregate upon removal of the suspending solvent phase during slip casting, will be present. These powders still exhibit sufficient agglomeration to induce occasional preferential sintering defects in the finished microstructure of the ceramic, resulting in local variations in density and potential stress concentration failure centers.
Further, many of today's newest and most promising ceramics such as silicon nitride are not processible by controlled hydrolysis of alkoxide solutions. For these ceramics, other methods of particle formation are required.
Experimental and theoretical evidence has shown that the size uniformity must be less than .+-.1 percent to promulgate ordered suspensions in which the particles pack into a close-packed hexagonal or face-centered cubic array of a wide range of particle volume fractions, .phi., which may be as low as .phi.=10 percent. Defect-free sintering should be much more probable from a green body with an ordered particle arrangement.
The method of the present invention is effective largely because the organometallic compounds are liquid solutions in which controlled hydrolysis produces uniform microsized particle precipitation. Alternative mechanization of the alkoxide hydrolysis reaction and precipitation process, using aerosolparticle formation and flow stream processing, provide a high degree of particle handling thereby avoiding agglomeration. Production of the organometallic precursor liquids as uniform size aerosol droplets and subsequent electrostatic charging and electromagnetic separation of these particles assures high uniform size and lack of agglomeration during subsequent reaction to form the finished molding powder. Dispersal and coating of sintering aids and grain growth limiting agents on the surface of the ceramic precursor particle during flow stream processing results in the production of truly single size particles in a continuous process.
The method of the present invention produces monosize ceramic precursor particles in free flight, one separated from another. The precursor particles are processed to a sinterable ceramic in flight. The ceramic precursor particle is uniformly coated with the sintering aid and grain limiting substances at the precursor stage. The process employs CVD (chemical vapor deposition) of the sintering aid during flight of the already formed ceramic particle.